1. Field of the Invention
The present invention relates to a hydraulic active suspension system for a vehicle such as an automobile or the like, and, more particularly, relates to a hydraulic active suspension system which effects vehicle height feedback control.
2. Prior Art
In a vehicle such as an automobile or the like, under the same conditions of external turbulences, such as, for example, lateral wind gusts exerted on a vehicle body, load surface undulations, inertial forces and the like, the behaviour of the vehicle body varies according to vehicle speed. That is, as the vehicle speed increases, the behaviour of the vehicle body becomes increasingly variable and the controllability and stability of the vehicle decreases.
In this connection, in the Japanese Utility Model Laying Open Publication No. Showa 62-185608 (1987), an active suspension system is described in which desired vehicle heights are set so that during a turn the desired roll angle of the vehicle body relative to its lateral acceleration is lowered in magnitude, as the vehicle speed increases.
In such an active suspension system as mentioned above, however, since the desired vehicle heights are set so that during a turn the desired roll angle relative to its lateral acceleration is lowered in magnitude as the vehicle speed increases, the same effect can be produced in a suspension system which uses vehicle height feedback control based upon a component proportional to the difference between a desired and an actual vehicle height and a component proportional to an integral value of the actual vehicle height. In such a system the coefficients for the two components are raised as the vehicle speed increases, and accordingly, the vehicle mode attitude feedback control tends to become unstable, causing hunting of the vehicle mode attitudes.